A can body of a drawn and ironed can (hereinafter referred to as the "DI can") which is a kind of the two-piece cans, is conventionally manufactured by means of a drawing and ironing forming (hereinafter referred to as the "DI forming") which comprises the steps of: drawing a circular sheet cut from a metal sheet by forcedly passing the circular sheet through a die with the use of a punch to form a cup, and then, redrawing and ironing the thus formed cup by forcedly and continuously passing the cup through a plurality of different dies with the use of another punch, thereby obtaining a can body having a thickness considerably thinner than that of the circular sheet.
A can body of a drawn and redrawn can (hereinafter referred to as the "DRD can") which is also a kind of the two-piece cans, is conventionally manufactured by means of a drawing and redrawing forming (hereinafter referred to as the "DRD forming") which comprises the steps of: drawing a circular sheet cut from a metal sheet by forcedly passing the circular sheet through a die with the use of a punch to form a cup, and then, redrawing the thus formed cup by forcedly passing the cup through another die with the use of another punch, thereby obtaining a can body.
The two-piece can such as the DI can or the DRD can as described above is light in weight because of the reduced thickness of the can body as compared with a so-called three-piece can, which comprises a top, a bottom, and a drum, prepared by soldering or welding, and free from leakage of the content because of the absence of a joint in the drum. For these advantages, there is an increasing demand for two-piece cans, and the range of uses thereof is expected to expand.
As a metal sheet for the two-piece can, it is the usual practice to use an electrolytic tin-plated steel sheet or an aluminum sheet. The demand for the two-piece can made of the electrolytic tin-plated steel sheet is expected to grow since the electrolytic tin-plated steel sheet is lower in cost than the aluminum sheet. The weight of deposited tin of the electrolytic tin-plated steel sheet is relatively slight. However, under the influence of the recent cost increase of electric power and tin, the increase in the manufacturing cost of the electrolytic tin-plated steel sheets and the two-piece cans made of the electrolytic tin-plated steel sheets is inevitable. There is therefore a strong demand for reducing the manufacturing cost of the electrolytic tin-plated steel sheets.
The reduction ratio of the thickness of the side wall of the can body of the DI can is so large as about 70% under the effect of ironing. It is therefore necessary to previously impart lubricity to the surface of the metal sheet. The tin-plating layer of an electrolytic tin-plated steel sheet provides an excellent lubricity to the steel sheet. However, if the weight of the deposited tin-plating layer is reduced with a view to reducing the manufacturing cost of the electrolytic tin-plated steel sheets, the following problems are caused in a DI can:
(1) The smaller weight of the deposited tin-plating layer results in a lower lubricity which in turn leads to a lower DI formability. As a result, during the passage of the can body through the dies in ironing, a sticking of the can body to the dies or a galling of the can body may occur under the effect of a friction heat and a working heat. Occurrence of such stricking or galling deteriorates corrosion resistance of the DI can.
(2) Rust tends to occur on the surface of the can body of the DI can after removing grease such as an external lubricant deposited onto the surface of the can body of the DI can during ironing.
As methods for solving the above-mentioned problems, the following methods are known:
(1) A method for manufacturing a can body of the DI can, disclosed in Japanese Patent Provisional Publication No. 51-63,787 dated Jun. 2, 1976, which comprises:
applying a precoating composition comprising any one of epoxy-phenol resin, epoxy-ureaformaldehyde resin and vinyl resin, or a precoating composition comprising any one of the above-mentioned resins and an internal lubricant, onto at least one surface of a metal sheet, to form a precoating film thereon, then partially curing the precoating film, and then subjecting the metal sheet having such a precoating film to the DI forming, thereby obtaining a can body of the DI can (hereinafter referred to as the "prior art 1").
(2) A method for manufacturing a can body of the DI can, disclosed in Japanese Patent Publication No. 60-4,753 dated Feb. 6, 1985, which comprises:
applying a precoating composition comprising epoxy-phenol resin or a precoating composition comprising epoxy-phenol resin and an internal lubricant onto the both surfaces of a surface-treated steel sheet to form a precoating film thereon, then partially curing the precoating film, then subjecting the surface-treated steel sheet having such a precoating film to the DI forming, and then completely curing the precoating film, thereby obtaining a can body of the DI can (hereinafter referred to as the "prior art 2").
(3) A precoated steel sheet for a DI can, disclosed in Japanese Patent Publication No. 58-18,229 dated Apr. 12, 1983, which comprises:
a steel sheet; and a precoating film formed on at least one surface of the steel plate, which is to be the outer surface of a two-piece can, by applying a precoating composition onto the at least one surface of the steel sheet and completely curing same, said precoating composition comprising (a) phenol modified epoxy resin comprising thermosetting phenol resin and bisphenol A type epoxy resin, and (b) olefinic hydrocarbon (hereinafter referred to as the "prior art 3").
(4) A precoating composition, disclosed in Japanese Patent Provisional Publication No. 57-168,961 dated Oct. 18, 1982, which comprises:
phenol resin, epoxy resin, vinyl chloride-vinyl acetate copolymer and polyvinyl chloride dispersed resin. The above-mentioned precoating composition may contain as required an internal lubricant such as wax, lanoline or petrolatum (hereinafter referred to as the "prior art 4").
(5) A precoated metal sheet for a two-piece can, disclosed in Japanese Patent Provisional Publication No. 62-275,172 dated Nov. 30, 1987, which comprises:
a metal sheet; a precoating film formed on one surface of the metal sheet, which is to be the outer surface of a two-piece can, by applying a precoating composition onto the one surface of the metal sheet and curing same, the above-mentioned precoating composition comprising a thermosetting coating material and a modified hydrocarbon wax as an internal lubricant, having a content ratio within the range of from 0.1 to 30 weight parts relative to 100 weight parts of a resin as a solid content in the thermosetting coating material; and another precoating film formed on the other surface of the metal sheet, which is to be the inner surface of the two-piece can, by applying another precoating composition onto the other surface of the metal sheet and curing same, the above-mentioned another precoating composition comprising only the above-mentioned thermosetting coating material, and containing no internal lubricant (hereinafter referred to as the "prior art 5").
The above-mentioned prior arts 1 to 4 have the following problems: Lubricity of the metal sheet during the DI forming cannot be increased sufficiently by means of any of the prior arts 1 to 4, and as a result, it is impossible to improve the DI formability of the metal sheet. In the prior art 1, furthermore, since curing of the precoating film is only partial, a precipitate from the precoating film not only deteriorates flavor and perfume of the content in the can, but also leads to a lower corrosion resistance of the DI can after painting. In the prior art 2, curing of the precoating film accomplished in two steps requires more complicated manufacturing processes and results in a higher manufacturing cost of the DI can.
According to the prior art 5, the problems involved in the prior arts 1 to 4 are solved, improving the DI formability of the metal sheet. However, the DI formability of the metal sheet can be improved in the prior art 5 only when the cup formed by drawing is subjected to ironing by causing same to forcedly and continuously pass through a plurality of dies with the use of a punch at a punching speed, i.e., a forming speed as slow as about 30 m/minute. A high forming speed of at least 100 m/minute causes the following problems: A high forming speed of at least 100 m/minute produces a high frictional heat and a high working heat during ironing between the dies and the outer surface of the can body and between the punch and the inner surface of the can body. The thus produced frictional heat and working heat cause softening of the precoating film formed on the inner surface of the can body, and sticking thereof onto the punch. As a result, when withdrawing the punch from the can body after ironing, the precoating film formed on the inner surface of the can body is peeled off from the inner surface of the can body, and gathers near the open end of the can body, thus producing a so-called build-up phenomenon of the precoating film. This seriously increases frictional resistance between the punch and the inner surface of the can body, and the can body moves along with the punch. As a result, the side wall of the can body buckles under the effect of the stripper, thus deteriorating strippability of the can body. In addition, peeling-off of, or damage to, the precoating film formed on the inner surface of the can body deteriorates corrosion resistance of the two-piece can.
Under such circumstances, there is a strong demand for the development of a precoated steel sheet for a two-piece can, which is excellent in DI formability, DRD formability and corrosion resistance, and the precoating film of which is never peeled off or damaged even when ironing the precoated steel sheet at a high forming speed of at least 100 m/minute, but such a precoated steel sheet for a two-piece can has not as yet been proposed.